PLASMA FOURTH S T A T OF M A T T E R Fixing equilibria at ultrahigh temperaturesa new techique with fantaftic possibilities A STAFF FEATURE
he plasma state is the source of highest continuously Higher temperaturn can be obtained, but they have only limited applications because of their short durationusually a few microseconds or milliseconds. Temperatures in a controlled nuclear reaction have been limited by the construction materials required in the reactor. The hottest chemical flames are obtained from high energy fuels such as aluminum powder burned in pure oxygen or from combustion of carbon subnitride, and flames produced from ordinary fuels fall far short of these energy levels. In contrast, the plasma in an electric arc discharge can be kept indefinitely at temperatures which range all the way from the levels of chemical flames to those which are hotter than wires exploded by high voltage discharge.
Tcontrollable temperatures available today.
Plasma-lonized
c
16
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Gas or Vapor
The original Greek word, plasma, meant molded or formed. Early arc scientists in Germany used the term to describe that part of the conduction column between anode and cathode fall regions of an arc discharge. This classical definition limited the term to include only an appreciably ionized gas or vapor which conducts electricity and is at the same time electrically neutral, fluid, hot, and viscous. The modern definition is less restrictive and includes ionized gases produced by shock waves and other devices in which there is no flow of
electrical current. I o d gases prescnr m glow die charges and other electronic phenomena which arc not hot are also plasmas by the modern drfinition. Today the term plasma generally refera to a more or less ionized gas. All gases are ionized to Some degree at high temperatures, but the temperature at which gas atoms begin to lose electrons is not necessarily high. Even some metals, notably cesium and potassium, have low enough ionization potentials that they are highly ionized at temperatures of 3000' to 3500' K. It is often convenient to emphasize electrical conductivity as an essential characteristic of plasma. An ionized gas may have an electrical conductivity equal to or even greater than that of solid copper. Plasma is present in any electrical discharge. All types of electric arcs are forms of plasma. The arc plasma is maintained by thermal ionization of the gas, which acts as the element of a resistance heater. Energy for the heater comes from the flow of electric current between the electrodes. Self-induced magnetic fields produced by the high current8 in the arc compress the plasma. These fields c a u e radial and axial pressure gradients in the arc. The axial gradient leads to plasma streams which transport material and heat to and from the electrodes. While most electronic plasma devices operate at low temperatures, the arc plasmas are hot and are generally of greater interest from the chemical point of view because of the temperatures which are attainable. At about 5000' K. thermal ionization prcduces enough ion pairs (a pair is a positive ion plus a free electron) to make the gas a pretty good conductor of electricity. As temperatures go higher-to 10,OOOoK. and a b v e the degree of ionization increases and plasma takes on the conductive properties of metallic materials. In contrast to metals, however, plasma is a compressible fluid, and thus it obeys not only the electromagnetic principles, but the laws of fluid mechanics as well. The study of the interaction of the various electromagnetic and mechanical forces is the basis for a new and complex science, magnetohydrodynamics. If we proceed even further, to a temperature between 10,OOO,OOOo and 100,000,000" K., we find that gases consist of a mixture of bare nuclei and free electrons. In a completely ionized hydmgen plasma, for instance, atoms are stripped of their electrons. Inside the s u n and in the solar corona, most helium atoms are believed to be stripped of both their orbital electrons. This state of matter is thought to exist in the innermost regions of the hottest stars. In this temperature range, thermonuclear reactions occur on a large scale. Fourth Stak of M m r
Plasma has often been called a fourth state of matter. Although quite common in the universe, the plasma state is not often found on earth. Astrophysicists were the fint group of scientists to show much interest in plasma, because most of the stars, including our own sun, are in the plasma state. More recently, however, with the interest in ultrahigh temperatures, chemists, physicists, VOL 5 5
NO. 1
JANUARY 1963
17
and engineers in many fields have become concerned with efforts to generate plasma economically and to develop practical applications. Unknown before the invention of the electric arc, man-made plasma has seen its greatest development in recent years, following the invention of the plasma jet. R. M. Gage’s constricted arc, gas fed torch, developed at Linde Co. in the early 195O’s, was the first commercial plasma jet. In studying the arc properties of rare gases, Gage had observed the flames produced when the arcs struck a water-cooled copper anode. By drilling a hole in the anode, he made the flaming arc pass through it, and gas flow under pressure produced a crude plasma jet. Finally, the hole was reduced in size to form a nozzle which constricted the arc and the plasma. With further refinements and modifications this became the patented Linde Plasma Arc Torch (U. S. Patents 2,806,124 and 2,858,411). Development of this modern plasma jet design ushered in a period of great interest and much activity in the plasma field. The flame from a controlled plasma arc torch has at least three big advantages over conventional oxygenfuel gas flames. -The gas used in arc plasma jets can be chosen according to desired properties. It can be inert or reactive, as required, and its composition is controllable as well as variable -Temperatures in the arc plasma flame can be many times higher than chemical flame temperatures -Heat intensiv or rate of heot transfer to an obiect, often more important than temperature per se, can exceed that of conventional burners by several times with proper choice of gas. Intensity of a nontransferred arc torch using hydrogen can be more than 100 times the-heat intensity of on oxyacetylene torch.
Plormo Chemistry
Shortly after the turn of the century, Birkeland and Eyde (4) developed an industrial process to synthesize nitric oxide from air using an electric arc. The process was used commercially in Norway until it was superseded by the Haber-Fink process (9), which was more economical and produced nitric acid directly from ammonia by oxidation. Schonherr (10) reported another nitrogen fixation proces in 1909. The high voltage or Hiils arc, a gas-stabilized arc with special geometric configurations, was developed for the hatior of atmospheric nitrogen. Baumann (3) of Chemischc Werke Hiils reported the development of a process foi synthesizing acetylene from saturated hydrocarbon^ using this high voltage arc. Since the arrival of commercial arc plasma generators a few yean ago, interest in the possibilities of chemical
More &tails of plasma jet chemistry appear in two 0 t h I&?ECpublications, both volume I. Process Dcsigr and Developmmt, page 161, discusses an analytical approach and on page 166, production of cyanogen. Fundmnmtals, pag. 52, describes thermodynamics of selected chemical systems. EDITOR’S NOTE:
I8
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
processes in the plasma jet has grown steadily. The plasma jet generator or plasma torch, with its ability to produce temperatures beyond the range of chemical combustion flames or the ordinary electric arc, has further stimulated the study of high temperature reactions. So far relatively few studies have been carried out on the use of the plasma jet in chemical processes. Phillips and Ferguson (9) in 1959 reviewed several plasma jet syntheses. Plasma chemistry is quite different from ordiwuy chemistry. At temperatures above 5000’ K., unusual things begin to happen. At these temperatures no solids exist at atmospheric pressure. Ions and electrons are common species, and even neutral particles have great kinetic enin highly excited states I
‘RE SOURCES L.
Kmakr
OR
Tmtp.
IUI
400 900 limited by constru
I
may react to give compounds which are unlikely to be formed in conventional chemical reactions. Production of compounds with a plasma jet depends on the temperature achieved and the quenching velocity of the compounds formed at these temperatures. Formation of compounds using a plasma jet as the high temperature source may be considered to occur in two steps. The first is the decomposition of the molecules either of a reactive plasma gas or of a gas fed into the plasma “flame,” into atoms or activated atoms. The second step is freezing out the chemical equilibria attained at the high plasma temperamres by fast quenching methods. Two different types of chemical reactions may be carried out successfully using plasma jet temperatures. Compounds may be decomposed into their elemen& or into less energetic compounds, or endothermic compounds may be formed by freezing the chemical equilibria attained at high temperatures using quenching methods. Formation of exothermic compounds is not possible, because the chemical equilibrium at the higher temperature is leu favorable for the compound than for its elements. Another possibility is the formation of free radicals to be used as intermediates in subsequent reactions. The hot gas stream from the plasma jet may take part in a reaction or it may act only as a heat source. An
inert gas such as argon or helium is used as a source of heat, while chemically reactive plasma gases such as hydrogen, nitrogen, or perhaps even oxygen, introduce reactive species into the system. Similarly, electrodes may be consumable or nonconsumable. Consumable electrodes introduce reactants into the plasma stream, whereas nonconsumable electrodes serve only to transmit the electrical current. Substances injected into the plasma stream may react among themselves or enter into chemical reactions with the plasma gas as well. Although the electric arc in various configurations has been used for quite some time in chemical synthesis, the plasma jet offers to open up new fields for arc chemistry. It has high temperatures, reasonably high efficiency, and potential for high pressure operation. Chemical processing at temperatures above 3000’ K., considered by many chemists and engineers to be not feasible technically and economically, now appears within the realm of possibility. Cyanogen from the Elemcnta. Recently Leutner ( 6 ) has succeeded in preparing cyanogen in a plasma jet according to the endothermic reaction:
2C
-.
+ NI
(CN),
- 71 kcaI./molc
Carbon vaporized from an ordinary graphite cathode
was allowed to react with a nitrogen jet, or with an
V O L 5 5 NO. 1 J A N U A R Y 1 9 6 3
19
argon jet into which nitrogen was fed. The result was the same in both cases. Up to 157, of the carbon input was converted into cyanogen. The economy of this process depends almost entirely on the cost of power consumed, and may be of industrial interest. Acetylene Production. Leutner and Stokes (7) tried three different methods for producing acetylene in a plasma jet: feeding powdered carbon into a hydrogen jet, using a methane jet, and feeding methane into the flame of an argon plasma jet. The third method was the most successful and produced a conversion of about 80% of the methane to acetylene according to the endothermic reaction:
2CH4
-
CZHZ
+ 3H2 -95.54
ammonia were fed into the flame of either an argon plasma jet or a nitrogen plasma jet. Methane serves as a carbon and hydrogen source while ammonia is the nitrogen source. Conversion to hydrocyanic acid and acetylene based on carbon (methane) input ranged from 60 to 75%, using either argon or nitrogen as plasma gases. By using nitrogen in excess, HCN formation is favored. With argon as the plasma gas, the formation of CzHz has preference. The preference depends on the quenching rate. Since acetylene is the more endothermic compound, as compared to hydrocyanic acid, faster quenching rates favor the formation of acetylene. Another route to HCN is reaction of water gas:
kcal./mole
C
+ HzO
The average calculated plasma jet temperature was about 12,000° K.
with generator gas :
Using a high intensity arc reactor, Baddour and Iwasyk at MIT studied the reactions of elemental carbon and hydrogen at temperatures above 2800' K . (2). Acetylene was the major product of the reaction.
according to the reaction :
C
+ azr
--+
CO f Hz
-+ CO
f Nz
2C $- H2 + CZH;, Acetylene concentration in the quenched sample reached 18.6 volume yo,and 23.8 volume yo (on a diluent-free basis) when a 66.7% helium dilution was used. The high intensity arc reactor was found to be superior to other high temperature reactors for studying reactions between carbon and hydrogen. Hydrogen Cyanide. Five different reactions were carried out in a plasma jet at the Research Institute of Temple University, using compounds of carbon, hydrogen, and nitrogen in different ratios as starting materials (5). Preparation of another endothermic compound, hydrocyanic acid, was the object. Hydrogen cyanide was produced from the elements according to the reaction :
2C f H2 f N 2 + 2HCN -60.2 kcal./mole A consumable ordinary graphite cathode was used as the carbon source, and hydrogen was fed into a nitrogen plasma jet. Over 50yc conversion to HCN, based on carbon input, was obtained. Only significant byproduct of the reaction was acetylene-I3 to 14%. Using carbon and ammonia, the reaction is the same, because ammonia decomposes quantitatively into nitrogen and hydrogen while passing through the plasma flame. Only difference in the process was that ammonia was fed into the nitrogen plasma jet instead of hydrogen and the vaporization rate of the carbon cathode was forced. HCN formation up to about 4Oye, based on carbon input, was obtained. By feeding methane into the nitrogen plasma jet, the carbon supply was increased and the hydrogen necessary for the HCN formation was supplied by the methane decomposition in the plasma flame. Conversion of up to 45% of the carbon to HCN was accomplished using a large excess of nitrogen. Switching to an assembly with nonconsumable cathode, stoichiometric mixtures of methane and 20
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
Stainless steel is coated on an aluminum plate. The particles, still in a plastic state, are defiosited at near-sonic velocity and cooled immediately bji jets of carbon dioxide
T o find whether this could be accomplished, stoichiometric mixtures of carbon monoxide and ammonia were fed into an argon plasma jet and into a nitrogen plasma jet. Gas analysis showed that only the ammonia was decomposed in the plasma flame. The carbon monoxide remained as a molecule, probably because the temperature was not high enough. Nitrogen-Hydrogen Reaction. In another experiment by the Research Institute of Temple UniL ersity (5),the formation of ammonia was tried : n'2
+ 3ff2
-
+
2Avf33 7 7 kcal./mole
Hydrogen was fed into a nitrogen plasma jet, but instead of ammonia, small amounts of hydrazine (heat of formation, - 1.7 kcal./mole) were formed. Other Nitrogen Compounds. Xtroyen dioxide was prepared by Stokes and Knipe ( 7 7 ) by injecting oxygen into a nitrogen plasma jet. Some nitric oxide
was also formed in the reaction, but the average conversion to oxides was only about 2y0 based on the oxygen. Later experiments (5) confirmed this poor rate of nitrogen fixation. Stokes and Knipe also succeeded in preparing nitrides of titanium and magnesium. The nitrides were produced by fluidizing the metal powders in nitrogen and feeding into a nitrogen plasma jet. Average conversion was 30y0 for titanium and 4Oy0 for magnesium. Under identical conditions, molybdenum and tungsten nitrides could not be prepared. I t should also be possible to produce tantalum nitride and zirconium nitride in the art. Aluminum Oxide Reduction. Use of the high temperatures of the plasma jet to carry out reductions is an attractive possibility. Preliminary studies ( 5 ) indicate that alumina can be decomposed into the elements by passing it through the plasma flame. A1203is strongly exothermic (heat of formation, f389.5 kcal./mole) . Fast quenching is necessary, however, to prevent the elements from recombining into alumina. In experiments with an argon plasma jet, small amounts of aluminum metal were produced, but conversion was very low. Analytical Approach. In a recent article, Anderson and Case ( I ) have stated that an analytical approach is desirable in order to realize fully the potential of the plasma torch for chemical reactions. Accordingly, an analysis of methane decomposition in a hydrogen plasma jet was made. The approach is said to be fruitful in supplying insight into what occurs within the reaction chamber and defining the practical limits of the methane decomposition process.
trode consumption and the presence of water vapor in the jets ruled out most potential applications. The advantage of arc constriction to stabilize the flame and concentrate power was recognized, but commercial use of plasma jets was not to come until Gage developed his constricted arc torch. In a gas-stabilized arc, all or part of the arc column is contained in a nozzle or chamber. Fluid enters the chamber under pressure, either tangentially to produce a swirling vortex, or axially to give a “sheath stabilized’’ arc. The flowing gas medium passes through the arc discharge, absorbing a significant portion of the input energy. The jet of plasma gas exits through the chamber orifice or nozzle. In the annular or toroidal plasma jet, the arc strikes radially from an inner to an outer electrode. The position of the arc path at various times resembles the spokes of a wheel. Axial gas flow through the annulus reduces heat flow to any one point on the electrodes.
Thermodynamics of Plasma Jet Synthesis
Because of growing interest in plasma jets and the potential utility of this device for commercial chemical syntheses, a group of workers at the Stanford Research Institute has attempted to define, by thermodynamic analysis of a typical system, the operating parameters for deriving maximum concentrations of some valuable product, produced in a quasiequilibrium process (8). The hydrogen-nitrogen-carbon ternary system was selected because it involves commercially important compounds-hydrogen cyanide, cyanogen, and acetylene. Data are also applicable to the carbon-hydrogen, carbon-nitrogen, and nitrogen-hydrogen binary systems. High Temperature Plasma Devices
There are a number of ways to get useful heat energy from plasma. Most of these are plasma jets (also called constricted arcs) in which the arc column is compressed by some means outside the arc itself. The Gerdien arc, which was the first plasma jet, used a water vortex to stabilize the arc. Plasma jets squirted out of both ends toward the electrodes. Although temperatures up to 50,000° K. could be produced by making the water vortex small to squeeze the arc, these devices were mainly of academic interest. Rapid carbon elec-
Nose cone Jar a missile is given a heat-resistant tungsten coating
The arc is rotated rapidly by a magnetic field. Although this design allows high pressure operation and is valuable in research projects, it is inefficient with respect to conversion of input power into heat. Most plasma jets developed up to now operate on direct current. For the future, alternating current appears to be the ultimate in power sources because of availability and system simplifications. Many problems must be overcome, however, before alternating current systems replace d.c. arc generators. Among a.c.’s shortcomings are high transformer costs, instability, high erosion rates on electrodes, and low efficiency. Use of a liquid as stabilizing fluid allows the temperature of the arc stream to be greatly increased. In such plasma jets, however, electrode consumption is rapid, and the plasma produced is highly contaminated. These are receiving less and less attention. The Gerdien arc, which has become mainly a laboratory curiosity, is a good example of this type of device. VOL. 5 5
NO. 1
JANUARY 1 9 6 3
21
The transferred arc plasma jet has been widely used in metal cutting applications. I t differs from most of the other devices (nontransferred arcs) in that the arc does not end at the nozzle, but continues on to the workpiece or to a conducting member at the front of the torch. The target is connected to the current source. Resistance heating of the extended arc as it strikes the workpiece furnishes about two-thirds of the heat produced. This method can give higher temperatures and greater heat transfer rates than the more common nontransferred arc. Two-stage plasma jets have proved to be useful in chemical synthesis and in aeronautical wind tunnel studies. The two-stage jet is an adaptation of the simpler plasma jet forms. Gases which cannot be passed through the arc because of corrosive effects or solids deposition are introduced into the gas stream in a second chamber which is on the downstream side of the electrical circuit. A system where gas from two separate electrodes is fed into a plenum chamber has also been designed. Although this gives very low contamination levels and high enthalpy values, the power efficiency appears to be low, around 357,. The high intensity arc, a special type of arc discharge, transfers most of the input energ). directly to the anode face. Anode material is rapidly vaporized, heated above 7000’ C., and comes from the arc as a stream of electrode vapor plasma which is called the “tail flame” of the arc. IYhile most plasma jets get compression of the arc column from some external source, the high intensity arc gets its constriction by increasing power input. Most of the other constricted arcs depend on cooling the outer portions of the arc column to decrease ionization and cut down conductivity in that portion of the plasma stream. This causes current density at the center of the column to increase because of the tendency for current to concentrate where temperature and conductivity are highest. This tb-pe of arc has been used as a source of high intensity light for such things as military searchlights. As a heat source it has been used in chemical processing of refractory materials and for heating liquids and gases. Solenoidal magnetic fields, shock tubes, focused electron beams, and the contacting of cesium vapor ~ i t ah heated metal surface have also been investigated as means of producing various forms of plasma. 4 t least five companies are offering plasma jet torches and auxiliary equipment for sale. They are Plasmadyne Corp. of Santa Ana, Calif., Thermal Dynamics Corp. of Lebanon, N . H.. Linde Co., a division of Union Carbide Corp., Metallizing Engineering Co. of \Vestbury, Long Island, W. Y . , and AVCO Research and Advanced Developrnent Div. of It’ilmington, Mass. Plasma Jet Applications
Most plasma jet uses fit into one of four categories. These are processing and fabrication of materials,
The
sma torch is used‘ f o r cutting metals
Qzcirt
low velocit)) Jumc uj’ helium plusnia
L ~ , Zieuh
50,000’F.
chemical reactors, materials evaluation, and aerodynamics and space propulsion. Currently most commercial applications are in the metalworking field. Most chemical processes are in the research and development stages. Growth of the missile and rocket programs is pushing the materials evaluation and aerodynamic uses to new heights. As originally introduced, the Linde transferred-arc torch was capable of cutting aluminum I1/z inches thick. This technique has been extended until 5-inch aluminum plate can be cut, and 4-inch thicknesses of stainless steel, copper, magnesium, and other metals can also be handled. The transferred arc is also successfully used in welding applications. Spraying or coating with refractory metals is another important application. ‘4dvantages of the plasma arc in this use are the extremely high temperature and the ability to operate in nonreactive atmosphere. One of the first established uses was in coating graphite rocket engine parts with a layer of tungsten. Applications of this tb-pe are still predominant in plasma-spray technology. The plasma arc can be used to plate or coat objects with materials that are high-melting, reactive, or both. In spraying such materials as metals, alloys, and refractory oxides or carbides, the materials are fed into the plasma flame in the form of either
wire or powder. In powder form the material is usually dispersed in a carrier gas. Plasmadyne Corp. developed a plasma-spray coating device for applying plastic materials, including Penton, nylon, Teflon, and most epoxies. This plasma jet operates at a relatively low temperature (3000’ F.) and takes advantage of use of inert gases to provide a nonreactive atmosphere. Using nitrogen, argon, or helium, it melts and sprays powders of the plastic materials without charring or polymerization. Applications range from corrosion- and ablation-resistant coatings to insulations and decorative coverings. Uses in the metallurgical field are also appearing. The induction plasma torch is used to grow large single crystals of pure metals and compounds. Recently, Linde Co. introduced a plasma arc melting furnace for steels and other metals. The plasma arc provides a clean source of intense heat. Temperatures above 30,000’ K. can be reached, and quality of metal produced is equal to vacuum melted metals. Work has been done in 25-pound capacity furnaces. Larger furnaces from 50 pounds to 2 tons capacity are expected to find widespread use in both large and small foundries and in research operations for steel mills. Development work is under way to use this principle in basic steelmaking furnaces ranging up to 10 tons capacity.
Most work in materials evaluation and aerodynamics has come as a result of the missile and space programs. A big factor in motivating the development of arc technology in the last 10 years has been the urgent need for facilities to evaluate materials under simulated re-entry conditions. Plasma generators using air as a working fluid are supplying the high temperature, high enthalpy conditions needed to simulate missile flight in a wind tunnel at supersonic speeds. Looking to the future, ion propulsion appears as a possible means of propelling space travelers to other planets. The heart of an ion propulsion engine is the plasma generator producing the ion stream. Potential Applications. In addition to the many chemical reactions possible in the plasma jet flame, much valuable information can be gained through research using the plasma torch as a tool. Available data on chemical and physical properties of materials at elevated temperatures are quite limited and sometimes are not reliable. Data are scarce on thermal conductivity, specific heats, strength of materials, and electrical conductivity at ultrahigh temperatures. Much is to be learned about kinetic behavior at high temperatures. Since the plasma jet vaporizes all substances, it makes many vapor phase processes possible. Crystals of refractory materials can be grown from the vapor phase, and monocrystalline or polycrystalline metals and compounds can be produced from melts. Vapor separation of ores and production of alloys are possibilities. The industrial application of many of the chemical syntheses seems not too remote. Cost calculations based on scale-up of research installations have shown estimates of cost no more than double the current market price for acetylene and hydrocyanic acid, and even lower for cyanogen. At this early stage, the figures look encouraging. The plasma jet should prove to be a versatile process tool as well as a valuable aid to increasing the knowledge of matter. LITERATURE CITED (1) Anderson, J. E., Case, L. K., IND.ENG.CHEM. PROCESSDESIGN DEVLOP.1, 161-5 (July 1962). (2) Baddour, R. F., Iwasyk, J. M., Ibid., 1, 169-76 (July 1962). (3) Baumann, P., Angew. Chem. B20, 257-9 (1948). (4) Edstrom, J. S., Electrochem. Industry 2, 399-400 (1904). (5) Grosse, A. V., Leutner, H. W., Stokes, C. S., “Plasma Jet Chemistry,” 1st Ann. Rept., Research Institute of Temple University, p. 16-30, Dec. 31, 1961. (6) Leutner, H. W., IND.ENG.CHEM.PROCESS DESIGNDEVELOP. 1,166-8 (July 1962). (7) Leutner, H. W., Stokes, C. S., IND. ENG. CHEM.53, 341-2 (May 1961). (8) Marynowski, C. W., Phillips, R. C., Phillips, J. R., Hiester. N. K., IND.END.CHEM.FUNDAMENTALS 1, 52-61 (February 1962). (9) Phillips, R. C., Ferguson, F. A., “High Temperature Technology,” pp. 192-7, McGraw-Hill, New York, 1960. (10) Schonherr, O., Electrotech. 2. 30, 365-9, 397-402 (1909). (11) Stokes, C. S., Knipe, W. W., IND. ENG. CHEM.52, 287-8 (April 1960).
Part.: can be made of or :oated with ultrahigh temfierature materials which preiriously have been unworkable. The material shown here is tungsten and the items are a rocket nozzle and liners, tubes, a cruciblr. and grid cages
